Cube corner sheeting having optically variable marking

ABSTRACT

Retroreflective sheeting having a structured layer of cube corner elements and an at least one optically variable mark therein, and methods of making same.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.14/114,938, filed on Oct. 31, 2013, now U.S. Pat. No. 9,463,601, whichis a National Stage Filing under 35 U.S.C. 371 of PCT/US2012/039022,filed May 23, 2012, which claims priority to U.S. Provisional PatentApplication No. 61/491,602, filed May 31, 2011, the disclosures of whichare incorporated by reference in their entirety herein.

BACKGROUND

Retroreflective materials have the ability to redirect light incident onthe material back toward the originating light source. This property hasled to the widespread use of retroreflective sheeting for a variety oftraffic and personal safety uses. For example, retroreflective sheetingis commonly employed in a variety of articles, such as, road signs,barricades, license plates, pavement markers, and marking tape, as wellas retroreflective tapes for vehicles and clothing.

Two known types of retroreflective sheeting are microsphere-basedsheeting and cube corner sheeting. Microsphere-based sheeting, sometimesreferred to as “beaded” sheeting, employs a multitude of microspherestypically at least partially embedded in a binder layer and havingassociated specular or diffuse reflecting materials (e.g., pigmentparticles, metal flakes, vapor coats, etc.) to retroreflect incidentlight. Due to the symmetrical geometry of beaded retroreflectors,microsphere based sheeting exhibits the same light return regardless oforientation (i.e., when rotated about an axis normal to the surface ofthe sheeting). Therefore, it is said that the distribution of lightreturned by beaded retroreflective sheeting is generally rotationallysymmetric. Thus when viewing or measuring the coefficient ofretroreflection (“R_(A)”, typically expressed in units of candelas perlux per square meter) at presentation angles from 0 to 360 degrees, orwhen measuring at orientation angles from 0 to 360, there is relativelylittle variation in the retroreflectivity of beaded sheeting. For thisreason, such microsphere-based sheeting has a relatively low sensitivityto the orientation at which the sheeting is placed on a surface. Ingeneral, however, such sheeting has a lower retroreflective efficiencythan cube corner sheeting.

Cube corner retroreflective sheeting, sometimes referred to as“prismatic” sheeting, typically comprises a thin transparent layerhaving a substantially planar first surface and a second structuredsurface comprising a plurality of geometric structures, some or all ofwhich include three reflective faces configured as a cube cornerelement. Cube corner retroreflective sheeting is commonly produced byfirst manufacturing a master mold that has a structured surface, suchstructured surface corresponding either to the desired cube cornerelement geometry in the finished sheeting or to a negative (inverted)copy thereof, depending upon whether the finished sheeting is to havecube corner pyramids or cube corner cavities (or both). The mold is thenreplicated using any suitable technique such as conventional nickelelectroforming to produce tooling for forming cube cornerretroreflective sheeting by processes such as embossing, extruding, orcast-and-curing. Known methods for manufacturing the master mold includepin-bundling techniques, direct machining techniques, and techniquesthat employ laminae. These microreplication processes produce aretroreflective sheeting with prismatic structures that have beenprecisely and faithfully replicated from a microstructured tool having anegative image of the desired prismatic structure.

Prismatic retroreflective sheeting, in contrast to beadedretroreflective sheeting, is generally rotationally non-symmetric. Thus,when viewing or measuring R_(A) at presentation angles from 0 to 360degrees, or when measuring at orientation angles from 0 to 360, there issignificant variation in the retroreflectivity of prismatic sheeting.Therefore, prismatic sheeting has a higher sensitivity to theorientation at which the sheeting is placed on a surface than beadedsheeting.

SUMMARY

In view of the progress in the field of computer-based desktoppublishing, scanning, and laser-marking, there is a need to minimize orprevent the unauthorized replication of security marks (e.g., securitymarks used in license plates). In at least some implementations, it maybe desirable to have security marks in license plates or sheetingobservable by the unaided eye when the viewer is head-on looking at thelicense plate or sheeting from a distance (e.g., in a range from about0.1 meter to about 10 meters).

Consequently, the present disclosure describes security marking for useon retroreflective sheeting. The security marking makes use of theasymmetrical properties of prismatic retroreflective sheeting to createvisible features.

In one aspect, the present disclosure describes retroreflectivesheeting, comprising: a structured layer comprising cube cornerelements; and at least one optically variable mark in the structuredlayer of cube corner elements; wherein the at least one opticallyvariable mark comprises at least first and second mark elements, thefirst optically variable mark element having a first visual feature, andthe second optically variable mark element having a second visualfeature different from the first visual feature.

In a second aspect, the present disclosure describes retroreflectivesheeting, comprising: a structured layer of cube corner elements; and atleast one optically variable mark in the structured layer of cube cornerelements; wherein the at least one optically variable mark comprises amark element path having a continuously varied visual feature along itslength.

In a third aspect, the present disclosure describes a method of formingretroreflective sheeting, the method comprising providing a mold havinga structured surface including a plurality of cube corner cavitiestherein; at least partially filling the plurality of cube cornercavities with a radiation curable resin; exposing the radiation curableresin to a first, patterned irradiation to provide a first energydensity level of irradiation to the radiation curable resin in a firstarea of the plurality of cube corner cavities, to form a first opticallyvariable mark element; exposing the radiation curable resin to a second,patterned irradiation to provide a second energy density level ofirradiation different from the first energy density level of irradiationto the radiation curable resin in a second area of the plurality of cubecorner cavities, to form a second optically variable mark element;exposing the radiation curable resin to a third irradiation to provide athird energy density level of irradiation to at least the radiationcurable resin in an area of the plurality of cube corner cavitiescontiguous with the first and second areas thereof, wherein the thirdenergy density level is different from the first and second energydensity levels, to provide retroreflective sheeting having a structuredlayer of cube corner elements and an optically variable mark thereincomprising the first and second mark elements; and separating theretroreflective sheeting from the mold.

In a fourth aspect, the present disclosure describes a method of formingretroreflective sheeting, the method comprising providing a mold havinga structured surface including a plurality of cube corner cavitiestherein; at least partially filling the plurality of cube cornercavities with a radiation curable resin; exposing a first portion of theradiation curable resin along a predetermined path to a first,continuously varied level of irradiation to provide an opticallyvariable mark element; and exposing at least a second portion of theradiation curable resin in a plurality of cube corner cavitiescontiguous with the mark element path to a second irradiation, toprovide retroreflective sheeting having a structured layer of cubecorner elements and an optically variable mark element therein; andseparating the retroreflective sheeting from the mold.

In this application, “at least partially filling” refers to coating atleast some individual cavities in a plurality of cavities in thestructured surface of a mold with a resin, such that the individualcavities in the plurality of cavities each have some amount of resinwithin them;

“Continuously varying” refers to a gradual increase or decrease in thevalue of a variable; (e.g., a “continuously varied” level of irradiationprogresses gradually, within machine tolerances, through a range ofenergy density values);

“Continuously varying energy density” or “continuously varied energydensity level” as used herein means that the energy emitted by anapparatus and/or absorbed by an area varies over at least one of time orspace (i.e., distance). This time or space interval can be any intervalthat is greater than the inherent tolerance or error of the apparatus.In other words, a “continuously varied energy density” is one that isintentionally altered over at least one of time or space. In oneexemplary instance, the energy emitted by the apparatus at a time 1differs from the energy emitted by the apparatus at a time 2. In oneembodiment, the time interval between time 1 and time 2 is about 1millisecond. In another exemplary instance, the energy absorbed by theresin in an area 1 differs from the energy absorbed by the resin in anarea 2. In some embodiments, the distance between area 1 and area 2ranges from about 0.3 millimeter to about 1 millimeter. In bothinstances, the differential between the energy emitted or absorbed atpoint 1 and point 2 can be positive (i.e., increased) or negative (i.e.,decreased). In some exemplary embodiments, the apparatus emitting theenergy is programmed to change the energy density emitted by theapparatus at some time interval. In some instances, this difference is agradual progression (i.e., one or more gradual changes). In someexemplary embodiments, the continuously varied energy density level hasa highest energy density value and a lowest energy density value, andthe difference between the highest energy density value and the lowestenergy density value is at least 0.1 Joule/cm².

“Cube corner cavities” refers to cavities in the structured surface of amold that typically have trihedral structures that have threeapproximately mutually perpendicular lateral faces meeting in a singlecorner;

“Different energy density” refers to a difference of at least 0.1Joule/cm² between a first energy density level of irradiation and asecond energy density level of irradiation.

“Land layer” refers to a layer disposed immediately adjacent to the baseof the cube corner elements;

“Mark element” refers to a component of an optically variable mark ofthe present disclosure.

“Mark element path” refers to a predetermined region that includes acomponent of an optically variable mark of the present disclosure. Amark element path includes at least one mark element, and can include aplurality of discrete and/or continuous mark elements.

“Optically variable mark” refers to a retroreflective mark exhibiting avarying appearance depending on, for example, the angle at which themark is viewed, or the type of light that is used to view theretroreflective mark (e.g., reflective light versus transmissive light,or visible versus non-visible light). An optically variable mark may becontinuous (e.g., an unbroken line) or discontinuous (e.g., a brokenline);

“Partially cured” refers to part of a radiation curable resin beingcured to such a degree that it will not substantially flow;

“Pattern” refers to a spatially varying appearance and is at least oneof a uniform or periodic pattern, a varying pattern, or a randompattern; in some embodiments, the pattern is a non-random pattern.

“Patterned irradiation” refers to at least one of irradiating throughtransparent regions of a mask, guiding a beam of light, guiding a beamof electrons, or projecting a digital image, to generate a pattern ofcuring in a radiation curable resin;

“Security mark” refers to an element on or in a retroreflective filmthat is surrounded by a background visual appearance. In manyembodiments, the security mark is an “island” feature surrounded by acontinuous background appearance. The security mark can changeappearance to a viewer as the viewer changes their point of view of thesecurity mark. A security mark may be continuous (e.g., an unbrokenline) or discontinuous (e.g., a broken line).

“Visible” refers to being apparent and identifiable (i.e., to ascertaindefinitive characteristics of) to the unaided human eye of normal (i.e.,20/20) vision, using light from within a wavelength range of 400 nm to700 nm. By “unaided”, it is meant without the use of a microscope ormagnifying glass.

The disclosed sheeting may be useful in a variety of applications suchas securing tamperproof images in security documents, passports,identification cards, financial transaction cards (e.g., credit cards),license plates, or other signage.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIGS. 1 to 4 are illustrations of exemplary embodiments ofretroreflective sheeting of the present description;

FIG. 5 is an illustration of an exemplary embodiment of retroreflectivesheeting of the present description, as viewed under differentobservation angle conditions;

FIG. 6 is an illustration of an exemplary embodiment of retroreflectivesheeting of the present description; and

FIG. 7 is an illustration of an exemplary apparatus and an exemplarymethod for producing a retroreflective sheet of the present description.

Like reference numbers in the various figures indicate like elements.However, it will be understood that the use of a number to refer to acomponent in a given figure is not intended to limit the component inanother figure labeled with the same number. Some elements may bepresent in identical or equivalent multiples; in such cases only one ormore representative elements may be designated by a reference number butit will be understood that such reference numbers apply to all suchidentical elements. Unless otherwise indicated, all figures and drawingsin this document are not to scale and are chosen for the purpose ofillustrating different embodiments of the description. In particular thedimensions of the various components are depicted in illustrative termsonly, and no relationship between the dimensions of the variouscomponents should be inferred from the drawings, unless so indicated.Although terms such as “top”, bottom”, “upper”, lower”, “under”, “over”,“front”, “back”, “outward”, “inward”, “up” and “down”, and “first” and“second” may be used in this disclosure, it should be understood thatthose terms are used in their relative sense only unless otherwisenoted. In particular, in some embodiments certain components may bepresent in interchangeable and/or identical multiples (e.g., pairs). Forthese components, the designation of “first” and “second” may apply tothe order of use, as noted herein (with it being irrelevant as to whichone of the components is selected to be used first).

DETAILED DESCRIPTION

The optically variable marks can be formed in a cube cornerretroreflective (prismatic) structure. The marks change appearance withobservation angle. Observation angle is the angle subtended by theviewing axis and the source axis, which intersect at the surface of theprismatic material. At small observation angles, the marks may appeardark relative to a bright non-marked field, or vice-versa. In someembodiments, as observation angle increases, the marks become similar inappearance to the non-marked field, and then become brighter than thenon-marked field. Alternatively, the marks become similar in appearanceto the non-marked field, and then become darker than the non-markedfield. Finally at large observation angles, in some embodiments, forexample more than ten degrees, the marks are no longer visible. Thevariable effects of the marks are easily seen by an observer, by holdinga light source close to the eyes, and then moving it to the side, thusincreasing the observation angle.

The marks also show optically variable effects which change withentrance angle. Entrance angle is the angle subtended by the source axisand an axis normal to the prismatic surface. In some embodiments, whenviewing a marked prismatic panel at a fixed, small observation angle,while changing the entrance angle by tilting the panel, the marked areaappears dark until nearly 90 degrees entrance, then changes to brightagainst a dark non-marked field.

By changing the conditions used to form the marks, it is possible tocontrol the angles at which the marks change from dark to bright to nonvisible. In some embodiments, the marks change from bright tonon-visible. Thus, marks with a variety of visual effects can be formed.

Further, it is possible to form several marks in the same region of aprismatic material, each mark having different optically variableeffects seen at a different range of angles. Thus, when the marks areviewed together across a range of observation angles, they vary indifferent ways. In one exemplary embodiment, a series of marks is formedat specific conditions, resulting in the appearance of motion asobservation angle is changed.

FIG. 1 illustrates an exemplary embodiment of retroreflective sheeting100, having optically variable mark 150 and background areas 160 and161. Optically variable mark 150 includes optically variable markelements 110, 112, 114, and 120. Each of the labeled areas (i.e., 110,112, 114, 120, 160, and 161) includes cube corner elements, and each ofthe labeled areas has at least one visual feature associated with thecube corner elements. Optically variable mark element 110 has a visualfeature (e.g., retroreflectance at a given observation angle) differentfrom the corresponding visual feature (i.e., retroreflectance at a givenobservation angle) associated with optically variable mark element 114.Optically variable mark elements 110 and 112 may be visuallydistinguishable from each other. Background areas 160 and 161 havevisual features (e.g., retroreflectance at a given observation angle)that are visually indistinguishable from each other, though fromdifferent observation angles, they may be visually distinguishable fromeach other.

Optically variable mark element 120 is shown in FIG. 1 as a broken linerunning through optically variable mark element 114, consisting ofalternating dots and dashes, wherein the visual feature (e.g.,retroreflectance at a given observation angle) is constant along thelength of optically variable mark element 120.

FIG. 2 illustrates an exemplary embodiment of retroreflective sheeting200, having optically variable mark 250 and background areas 260 and261. Optically variable mark 250 has contiguous optically variable markelements 215 and 216. In the embodiment shown, each of the labeled areas(i.e., 215, 216, 260, and 261) includes cube corner elements, and eachof the labeled areas has at least one visual feature associated with thecube corner elements. Optically variable mark element 215 has a visualfeature (e.g., retroreflectance at a given observation angle) differentfrom the corresponding visual feature (i.e., retroreflectance at a givenobservation angle) associated with optically variable mark element 216.Background areas 260 and 261 have visual features (e.g.,retroreflectance at a given observation angle) that are visuallyindistinguishable from each other, though from different observationangles, they may be visually distinguishable from each other.

FIG. 3 illustrates an exemplary embodiment of retroreflective sheeting300, having optically variable mark 350 and background areas 360 and361. Each of optically variable mark 350 and background areas 360 and361 includes cube corner elements, and each of the labeled areas has atleast one visual feature associated with the cube corner elements.Optically variable mark 350 is a single optically variable mark elementhaving a continuously varied visual feature along its length (i.e., fromthe point identified by label 315 to the point identified by label 316).

FIG. 4 illustrates an exemplary embodiment of retroreflective sheeting400, having optically variable mark 450 and background area 460.Optically variable mark 450 has a linear sequence of optically variablemark elements 1 to 13, each optically variable mark element having anassociated visual feature (generally indicated by gray scale shading)that is visually distinguishable from the corresponding visual featureassociated with adjacent optically variable mark element(s).

Referring again to FIG. 4, the arrangement of optically variable markelements in optically variable mark 450 in a linear sequence provides an“appearance of motion” effect under some conditions. For example, inembodiments where the associated visual feature is retroreflectance at agiven observation angle, and where the optically variable mark elementsare arranged in a sequence such that each successive optically variablemark element requires an increase in observation angle to achievemaximal retroreflectance, a viewer can direct a light source (e.g., aflashlight) at the reflective sheeting and sweep the light sourcethrough a range of observation angles selected to include maximalretroflectance for each of the optically variable mark elements, and theviewer will observe an apparent motion of maximal retroreflectance alongthe optically variable mark (i.e., maximal retroreflectance will appearto progress from one optically variable mark element to the next). Atypical range of observation angles for observing maximalretroreflectance includes 0.5 degree to 15 degrees. The range ofobservation angles is readily achieved by a viewer positioned about 10feet away from the retroreflective sheeting and extending an arm to oneside (while holding the light source in the hand of the extended arm),although other distances may also be used.

FIG. 5 provides an exemplary illustration of an “appearance of motion”effect, using a linear sequence of 3 optically variable mark elements(“segments”) 5A, 5B, and 5C. An appearance of motion can be achieved,for example, when the observation angle required for achieving a peakretroreflectance level in any given segment increases sequentially fromsegments 5A to 5C, in the direction indicated by arrow 501. In thearrangement of segments, a process of sweeping a light source through arange of increasing observation angles OA1 to OA4 (OA1<OA2<OA3<OA4)results in a sequence of peak retroreflectance levels appearing to movefrom one segment to the next, in the direction of arrow 501. Atobservations angles OA1, the visual to observed retroreflectance 5A1 isbrighter than observed retroreflectance 5B1 and 5C1. Progressing fromOA1 to OA2, the observed retroreflectance gradually becomes brighter forsegments 5B and 5C, until all three segments appear to be bright, and itwill appear to an observer that the brightness has moved into segments5B and 5C. The selection of visual features can be adjusted so that apeak brightness appears to migrate from one segment to the next (e.g.,in FIG. 4 a peak brightness may move sequentially from segment tosegment), or to extend across a range of segments (e.g., in FIG. 4 apeak brightness may gradually extend across the full range of segments 1to 13).

An optically variable mark of the present description includes at leastone visual feature. In some embodiments, optically variable marksinclude visual features independently selected from the group consistingof retroreflectance at a given orientation, entrance or observationangle, brightness at a given orientation, entrance or observation angle,whiteness at a given orientation, entrance or observation angle, androtational symmetry. These visual features can be created by having afirst portion that includes a first light divergence profile and thesecond portion includes a second light divergence profile, and the firstlight divergence profile is visually different than the second lightdivergence profile. The term “divergence profile” is described in U.S.Pat. No. 4,775,219 (Appledorn), the description of which is incorporatedherein by reference. In some embodiments, these visual features arecreated by a first portion that includes a first set of cube cornerelements having a first cube size and the second portion that includes asecond set of cube corner elements having a second cube size thatdiffers from the first cube size. In some embodiments, these visualfeatures are created by a first portion that includes a first set ofcube corner elements having a first degree of cant and a second portionthat includes a second set of cube corner elements having a seconddegree of cant that differs from the first degree of cant. In otherembodiments, for example, these visual features are created by having afirst portion that includes a first set of cube corner elements and thesecond portion includes a second set of cube corner elements, where thefirst and second sets of cube corner elements are truncated cube cornerelements and full cube corner elements, respectively.

Other embodiments of visual features arranged in a sequence or otherconfiguration can be provided by appropriate selection of patternedirradiation during the microreplication process.

The optically variable marks are formed using a radiation curablematerial during a “continuous cast and cure” microreplication process. Aradiation curable resin is coated onto a cube corner mold so as to atleast partially fill the cube corner cavities. A directed light source(e.g., an ultraviolet laser beam) is then used to irradiate theresin/mold in an imagewise fashion. The areas of resin receiving laserradiation are partly cured. Additional radiation curable resin is coatedonto a carrier web, which is then applied, coated side to the mold,using a nip roll. Additional curing radiation then irradiates the resinthrough the carrier web. The carrier web with cured cube cornersattached is then removed from the mold, becoming a finished cube film.Optionally additional curing radiation can then be applied to the cubefilm. Optionally the cube film can also be passed through an oven.

FIG. 7 illustrates an embodiment of apparatus 700 that includes moldingtool roll 725, having a microstructured surface having a plurality ofcavities 727. Plurality of cavities 727 is at least partially filledwith radiation curable resin 730, delivered from die 750 via roll 724.As molding tool roll 725 is rotated, radiation curable resin 730 passesunder guided radiation source 760. Irradiated regions of radiationcurable resin 730 are at least partially cured by first irradiation 740,while non-irradiated regions of radiation curable resin 730 are notcured, forming partially cured resin 731 comprising a pattern ofirradiated regions and non-irradiated regions.

In the exemplary embodiment illustrated in FIG. 7, radiation curableresin 730′ is coated onto a major surface of overlay film 721 (suppliedfrom roll 722) which is then nipped against partially cured resin 731 onthe microstructured surface of molding tool roll 725 (using roll 723),forming a partially cured composite 733 that is subsequently irradiatedby second irradiation source 741 to form more fully cured composite 734.More fully cured composite 734 is separated from molding tool roll 725at roll 723′, and passes under third irradiation source 742 toretroreflective sheeting having structured layer of cube corner elements735 and an optically variable mark in the structured layer of cubecorner elements. For convenience, the retroreflective sheeting can bewound onto take-up roll 722′.

In some embodiments of the current disclosure, plurality of cavities 727is partially filled with radiation curable resin 730 and passed underradiation source 760, followed by addition of radiation curable resin730′ and irradiation by second irradiation source 741, resulting information of composite cube corner elements. Examples of producingcomposite cube corner elements are described in U.S. Patent ApplicationNo. 61/491,554, entitled “RETROREFLECTIVE ARTICLES HAVING COMPOSITECUBE-CORNERS AND METHODS OF MAKING”, filed on the same date as theinstant application, the disclosure of which is incorporated herein byreference.

In some embodiments of methods of the present disclosure, a patternedirradiation can be provided by any of irradiating through transparentregions of a mask, guiding a beam of light, guiding a beam of electrons,or projecting a digital image, examples of which are also described inU.S. Patent Application No. 61/491,616, entitled “METHODS FOR MAKINGDIFFERENTIALLY PATTERN CURED MICROSTRUCTURED ARTICLES”, filed on thesame date as the instant application, the disclosure of which isincorporated herein by reference.

In some exemplary embodiments of the method illustrated in FIG. 7,radiation curable resins 730 and 730′ may be the same as each other. Insome other exemplary embodiments, radiation curable resins 730 and 730′may be different from each other.

Overlay film 721 is supplied from roll 722. Depending on the flexibilityof overlay film 721, a carrier layer (not shown) may be provided as abacking for overlay film 721. The carrier layer may become incorporatedinto the retroreflective article, (e.g., the carrier layer may be firmlyadhered to overlay film 721), or the carrier layer may be removablyassociated with overlay element 721 and removed after irradiation 742.

The finished retroreflective sheeting, viewed from the overlay filmside, has high retroreflectivity. For example, the coefficient ofretroreflection R_(A) may be greater than 600 cd/lx/sqM at 0.2 degreesobservation and 4 degrees entrance. R_(A) cannot easily be measured onthe narrow areas in embodiments where a directed laser beam used to forman optically variable mark (i.e., “laser marked areas”), but these areasare visually darker at small entrance angles, for example at 0.2 degreeobservation or less, and thus they have lower R_(A) values. As theobservation angle is increased, the laser marked areas increase inbrightness until they are much brighter than the adjacent non-markedareas. Thus, the laser marked areas provide the optically variableeffect of “switching” from dark to bright as a light source held nearthe eye and then moved to the side. Additional more complex opticallyvariable effects can also be produced by controlling the energy density(“ED”, Joules/cm²) of laser energy applied to laser marked areas. Wehave found that when higher laser ED is used, then the resulting marksappear darker at small observation angles, “switch” to bright at alarger observation angle, and remain bright to a larger observationangle, relative to marks made at lower ED.

In some embodiments, the laser beam can be directed using a 2D scannersystem controlled by a computer. Thus the process of forming anoptically variable mark is highly flexible and is useful for featuressuch as warranty marks and special customer logos. The marking processhas the desirable attribute that no added materials are needed. Further,the marks are part of the structure of the cubes, and thus are difficultto copy or alter.

In some embodiments, the energy density of the patterned irradiationexposed to the radiation curable resin is continuously varied. Exemplarymethods of continuously varying the energy density of a laser beaminclude use of a beam modulator, such as an acousto-optic modulator anelectro-optic modulator, and a circularly variable neutral densityfilter. Other embodiments would be apparent to one of ordinary skill inthe art.

In some embodiments, optically variable marks of the present disclosureare useful as security markings. The optically variable marks caninclude security markings formed by asymmetrical reflectance propertiesof prismatic films to create visible features that vary with viewingangle. The security mark and the surrounding area of the retroreflectivefilm have light reflectance patterns that do not “match up”. Thus thesecurity mark and the surrounding area have different retroreflectivityand different visual appearance at some viewing angles. Preferredsecurity features are difficult to copy by hand and/or by machine or aremanufactured using secure and/or difficult to obtain materials.

Suitable radiation curable resins for forming retroreflective sheetingof the present description include cross linked acrylates such asmultifunctional acrylates or epoxies and acrylated urethanes blendedwith mono- and multifunctional monomers. Further, a structured layer ofcube corner elements may be cast onto plasticized polyvinyl chloridefilm for more flexible retroreflective sheeting. These polymers may bepreferred, for example, because of thermal stability, environmentalstability, clarity, excellent release from the tooling or mold, andcapability of receiving a reflective coating. Retroreflective sheetingmay be prepared by casting a structured layer of cube corner elementsdirectly onto a film (see, e.g., U.S. Pat. No. 5,691,846 (Benson), thedescription of which is incorporated herein by reference).

The cube corner elements can be formed on a polycarbonate film about 0.5mm thick having an index of refraction of about 1.59. Useful materialsfor making retroreflective sheeting are preferably materials that aredimensionally stable, durable, weatherable, and readily formable intothe desired configuration. Generally any optically transmissive materialthat is formable, typically under heat and pressure, can be used. Thesheeting can also include colorants, dyes, UV absorbers or separate UVabsorbing layers, and other additives as needed. A backing layer sealingthe cube corner elements from contaminants can also be used, togetherwith an adhesive layer. Alternatively, a specularly reflecting coating(e.g., a metallic coating) may be used.

The optically variable mark can be any useful mark including a shape, afigure, a symbol, a design, a letter, a number, alphanumeric character,and indicia, for example. The optically variable mark may comprisediscrete elements forming a pattern or may be continuous.

EMBODIMENTS

Item 1. A method of forming retroreflective sheeting, comprising:

providing a mold having a structured surface including a plurality ofcube corner cavities therein;

at least partially filling the plurality of cube corner cavities with aradiation curable resin;

exposing the radiation curable resin to a first, patterned irradiationto provide a first energy density level of irradiation to the radiationcurable resin in a first area of the plurality of cube corner cavities,to form a first optically variable mark element;

exposing the radiation curable resin to a second, patterned irradiationto provide a second energy density level of irradiation different fromthe first energy density level of irradiation to the radiation curableresin in a second area of the plurality of cube corner cavities, to forma second optically variable mark element;

exposing the radiation curable resin to a third irradiation to provide athird energy density level of irradiation to at least the radiationcurable resin in an area of the plurality of cube corner cavitiescontiguous with the first and second areas thereof, wherein the thirdenergy density level is different from the first and second energydensity levels, to provide retroreflective sheeting having a structuredlayer of cube corner elements and an optically variable mark thereincomprising the first and second mark elements; and

separating the retroreflective sheeting from the mold.

Item 2. The method of item 1, wherein each of the first, patternedirradiation and the second, patterned irradiation independentlycomprises at least one of irradiating through transparent regions of amask, guiding a beam of light, guiding a beam of electrons, orprojecting a digital image.Item 3. The method of item 1, wherein the radiation curable resin is afirst radiation curable resin, and the method further comprises applyinga second radiation curable resin over the first and second opticallyvariable mark elements prior to the third irradiation.Item 4. The method of any preceding item, further comprising applying anoverlay film over the radiation curable resin.Item 5. The method of any preceding item, wherein the mold istransparent to at least one of the first, patterned irradiation, thesecond, patterned irradiation, or the third irradiation.Item 6. The method of any preceding item, wherein the first energydensity level and the second energy density level have an absolutedifference of at least 0.1 Joule/cm².Item 7. A method of forming retroreflective sheeting, comprising:

providing a mold having a structured surface including a plurality ofcube corner cavities therein;

at least partially filling the plurality of cube corner cavities with aradiation curable resin;

exposing a first portion of the radiation curable resin along apredetermined path to a first, continuously varied level of irradiationto provide an optically variable mark element; and

exposing at least a second portion of the radiation curable resin in aplurality of cube corner cavities contiguous with the mark element pathto a second irradiation, to provide retroreflective sheeting having astructured layer of cube corner elements and an optically variable markelement therein; and

separating the retroreflective sheeting from the mold.

Item 8. The method of item 7, wherein the patterned irradiationsindependently comprises at least one of irradiating through transparentregions of a mask, guiding a beam of light, guiding a beam of electrons,or projecting a digital image.

Item 9. The method of item 7, wherein the radiation curable resin is afirst radiation curable resin, and the method further comprises applyinga second radiation curable resin over the optically variable markelement prior to the second irradiation.

Item 10. The method of any one of items 7 to 9, further comprisingapplying an overlay film over the radiation curable resin.

Item 11. The method of any one of items 7 to 10, wherein the mold istransparent to at least one of the first, patterned irradiation, or thesecond irradiation.

Item 12. The method of any one of items 7 to 11, wherein thecontinuously varied energy density level has a highest energy densityvalue and a lowest energy density value, wherein the difference betweenthe highest energy density value and the lowest energy density value isat least 0.1 Joule/cm².Item 13. Retroreflective sheeting, comprising:

a structured layer comprising cube corner elements; and

at least one optically variable mark in the structured layer of cubecorner elements;

wherein the at least one optically variable mark comprises at leastfirst and second mark elements, the first optically variable markelement having a first visual feature, the second optically variablemark element having a second visual feature different from the firstvisual feature.

Item 14. The retroreflective article of item 13, wherein the structuredlayer of cube corner elements is disposed on a land layer in a rangefrom greater than 0 micrometer up to 150 micrometers.

Item 15. The retroreflective sheeting of item 13, wherein the firstvisual feature and the second visual feature are independently selectedfrom: retroreflectance at a given orientation, entrance or observationangle; brightness at a given orientation, entrance or observation angle;whiteness at a given orientation, entrance or observation angle; androtational symmetry.Item 16. A security article comprising the retroreflective sheeting ofitem 13.Item 17. The security article of item 16, wherein the optically variablemark is selected from the group consisting of a shape, figure, symbol,design, letter, number, alphanumeric character, indicia, andcombinations thereof.Item 18. The retroreflective sheeting of item 16, wherein the layer ofcube corner prisms further comprises a background visual appearancevisually distinguishable from the optically variable mark.Item 19. Retroreflective sheeting, comprising:

a structured layer of cube corner elements; and

at least one optically variable mark in the structured layer of cubecorner elements;

wherein the at least one optically variable mark comprises at least onevisual feature and wherein the at least one visual feature includes avisually distinguishable continuous variation within at least a portionof the optically variable mark.

Item 20. The retroreflective sheeting of item 19, wherein the structuredlayer of cube corner elements is disposed on a land layer having athickness greater than 0 micrometer and up to 150 micrometers.

Item 21. The retroreflective sheeting of item 19, wherein the at leastone visual feature is selected from: retroreflectance at a givenorientation, entrance or observation angle; brightness at a givenorientation, entrance or observation angle; whiteness at a givenorientation, entrance or observation angle; and rotational symmetry.Item 22. The retroreflective sheeting of item 19, wherein the at leastone optically variable mark is selected from the group consisting of ashape, figure, symbol, design, letter, number, alphanumeric character,indicia, and combinations thereof.Item 23. The retroreflective sheeting of item 19, wherein the layer ofcube corner prisms further comprises a background visual appearancevisually distinguishable from the optically variable mark.Item 24. Retroreflective sheeting made according to the method of anyone of items 1 to 12.Item 25. A vehicle license plate comprising the retroreflective sheetingof any one of items 13 to 24.Item 26. A sign comprising the retroreflective sheeting of any one ofitems 13 to 24.

EXAMPLES Test Methods

Measuring Observation Angle at which the Optically Variable Mark BecomesBright or not Visible

Samples of retroreflective sheeting comprising optically variable marksprepared as described in Examples 1-5 were viewed, in a darkened room,using a flashlight (obtained from MAG Instrument, Ontario, Calif., underthe trade designation “MINI MAGLITE”). The prismatic surface of theretroreflective sheeting was viewed along an axis approximately normalto the prismatic surface, and from a distance of about 3 meters. Theflashlight was aligned with and close to a viewer's eye (observationangle of about 0°) and subsequently moved to the side, therebyincreasing the observation angle. When the optically variable markappeared fully bright, the distance from the viewer's eye to flashlightwas measured and observation angle calculated. The flashlight was thenmoved further from the viewer's eye until the optically variable markwas no longer visible. The distance from the viewer's eye to theflashlight was recorded, and the observation angle was calculated.

The calculation of observation angle was according to the followingequation:observation angle=arctan [(distance from eye to flashlight)/(distancefrom eye to sheeting)]Materials

BAED bisphenol-A epoxy diacrylate obtained from Cytec Industries Inc.,Smyrna, GA, under the trade designation “EBECRYL 3720”. DMAEADimethylaminoethyl acrylate, obtained from Cytec Industries Inc. TMPTATrimethylolpropane triacrylate, obtained from Cytec Industries Inc. HDDA1,6-hexanediol diacrylate, obtained from Cytec Industries Inc. TPO(2,4,6 trimethylbenzoyl) diphenylphosphine oxide, a photoinitiator,obtained from Sigma-Aldrich, St. Louis, MO. EAA Ethylene acid acrylate,obtained from Dow Company, Midland, MI, under the trade designation“PRIMACOR 3440”.Preparation of Composition 1

A first radiation-curable resin (Composition 1) was prepared bycombining 25 wt. % BAED, 12 wt. % DMAEA, 38 wt. % TMPTA, 25 wt. % HDDA,and 0.5 pph (parts per hundred) TPO.

Illustrative Examples 1-3

An overlay film was made by extruding an EAA film at a thickness of 0.06mm (2.5 mil) onto a 0.05 mm (2 mil) corona treated polyethyleneterephthalate (PET) carrier film according to the following description:pellets of EAA were fed into a 1.9 cm (0.75 in.) single screw extruder(obtained from C. W. Brabender Instruments Inc., South Hackensack, N.J.)with temperatures set at 140° C. (284° F.) for zone 1 and ramped up to175° C. (347° F.) at the extruder exit and die, resulting in a melttemperature of about 175° C. (347° F.). As the molten resin exited theextruder, it passed through a conventional horizontal film die (obtainedfrom Extrusion Dies Industries LLC, Chippewa Falls, Wis., under thetrade designation “ULTRAFLEX-40”) and was cast onto the PET carrierfilm. The PET carrier film was traveling at about 36 meters/min (120ft/min). The resulting molten overlay film on the PET carrier film wasrun between a rubber roll/chilled steel roll nip to solidify the moltenresin into a layer. The EAA surface was corona treated at an energylevel of about 1.0 J/cm².

The following description for the preparation of Examples 1-3 refers toapparatus 700 as generally shown in FIG. 7. A retroreflective sheetingwas prepared by coating a first portion of Composition 1 onto mold 725using conventional coating die 750 as generally shown in FIG. 7. Die 750was positioned to provide Composition 1 between mold 725 and rubbercoated nip roll 724. Mold 725 had a structured surface includingplurality of cube-corner cavities 727. Mold 725 was heated to about 180°F. (82° C.) and rotated at a speed of about 50 fpm (15.2 m/min).Composition 1 (730) partially filled the cube-corner cavities to about60% in volume, providing partially filled partial cube-corner structures731. An optically variable mark was formed by irradiating a portion ofpartially filled partial cube-corner structures 731 while in contactwith mold 725. The irradiated portion of the partially filled partialcube-corner structures were crosslinked with Nd:YVO₄ UV laser 760(obtained from Coherent Inc., Santa Clara, Calif., under the tradedesignation “AVIA 355-28”) having a wavelength of 355 nm and a spot sizeof 3.5 mm, with a mirror positioned at a distance of about 600millimeters from the tool surface. The mirror was oscillated to move thelaser beam across the surface (scanning speed) at a rate of 600millimeters per second in a generally sinusoidal pattern. The averagelaser power (in Watts (W)) and energy density varied in each of Examples1-3, as shown in Table 1, below.

TABLE 1 Examples Average laser power (W) Energy density (J/cm²) Example1 8.5 0.26 Example 2 15 0.72 Example 3 30.4 1.45

A second portion of Composition 1 (730′) was simultaneously cast ontothe EAA side of overlay film 721, supplied from roll 722. The coatedoverlay film was then nipped (using roll 723) against the mold 725containing the partially filled partial cube-corner structures 731, atleast some of which had been previously crosslinked. The resin coated onthe overlay film completely filled the unfilled portion of thecube-corner cavities, and the composite construction 733 was curedthrough the overlay film 721 to form a retroreflective sheeting 734,using two UV lamps 741 (obtained from Fusion Systems, Rockville, Md.,under the trade designation “FUSION D”) set at 600 W/in., and also usingdichroic filters (not shown) in front of the UV lamps. Theretroreflective sheeting 734 was separated from the mold 725 at roll723′ and subsequently irradiated by UV lamp 742 (“FUSION D”) operatingat 100% to provide a UV irradiation post-cure through the compositecube-corner structures 735. The retroreflective film 734 was then passedthrough an oven (not shown) set at about 130° C. (265° F.), and woundonto roll 722′.

The resulting cube-corner structures 735 had three sets of intersectinggrooves with a pitch of 3.2 mils (81 micrometers). The intersectinggrooves formed a cube-corner base triangle with included angles of 61°and a cube-corner element height of 1.95 mil (50 micrometers). Theprimary groove spacing is defined as the groove spacing between thegrooves which form the two 61° base angles of the base triangle.

The retroreflective sheetings prepared as described above comprisedoptically variable marks created at different energy densities. Theobservation angles at which the optically variable marks appeared brightor not visible were measured as described above. Results are reported inTable 2, below.

TABLE 2 Observation angle (°) Examples Mark bright Mark not visibleExample 1 1.2 2.6 Example 2 2.9 9 Example 3 4.8 >16

Example 4

A retroreflective sheeting comprising an optically variable mark wasproduced as generally described in Examples 1-3, except that a differentlaser was used (obtained from Coherent Inc. under the trade designation“AVIA 355-7”). A two-dimensional (2D) scanner for 355 nm wavelength(obtained from Cambridge Technology Inc., Lexington, Mass., under thetrade designation “PROSERIES SCAN HEAD”) and associated software wereused to control the shapes made with the laser beam, and also thevelocity of the laser on the resin and/or on the mold. A zoom beamexpander (obtained from Edmund Optics Inc., Barrington, N.J., under thetrade designation “MODEL NT64-420”) was installed to reduce the laserraw beam diameter from about 3.5 mm to about 1.0 mm to control theenergy density. The average laser power was 7 W. The optically variablemark generally comprised a first 612 and a second 610 intertwinedsinusoidal waves as illustrated in FIG. 6. The first sinusoidal wave 612was formed at a beam velocity of 400 mm/s and an energy density of 1.75J/cm². The second sinusoidal wave 610 was formed at a beam velocity of800 mm/s and an energy density of 0.88 J/cm².

The retroreflective sheeting prepared as described above included anoptically variable mark comprising first 612 and second 610 intertwinedsinusoidal waves created at different energy densities. Theretroreflective sheeting was viewed at a specific observation angle(calculated as described above) and whether the first and/or secondsinusoidal waves appeared bright or not visible was noted. Results arereported in Table 3, below.

TABLE 3 Observation angle (°) 1 3 5 First sinusoidal wave Bright BrightNot visible Second sinusoidal Bright Not visible Not visible wave

Example 5

A retroreflective sheeting comprising an optically variable mark wasproduced as generally described in Example 4, except the average laserpower was about 6 W, the pulse rate was about 70 kHz and the beam sizewas about 1.75 mm in diameter. An optically variable mark 450 comprisingthirteen discrete segments was produced by controlling the energydensity applied to each segment as shown in Table 4, below. The energydensity was varied by varying scanning speed.

TABLE 4 Segments Scanning speed (mm/s) Energy density (J/cm²) Segment 1300 1.3 Segment 2 350 1.1 Segment 3 400 1.0 Segment 4 450 0.89 Segment 5500 0.80 Segment 6 550 0.73 Segment 7 600 0.67 Segment 8 650 0.62Segment 9 700 0.57 Segment 10 750 0.53 Segment 11 800 0.50 Segment 12850 0.47 Segment 13 900 0.44

Example 6 (Prophetic Example)

A retroreflective sheeting could be prepared as generally described inExamples 1-3, except that a continuously variable beamsplitter could beplaced between the linearly polarized laser source and the 2D scanner,to modulate the energy density of the laser beam on the partially filledpartial cube-corner structures 731. The continuously variablebeamsplitter could comprise a half-wave waveplate and a polarizingbeamsplitter cube. An optically variable mark could be created byrotating the optical axis of the half-wave waveplate about thepropagation direction of the laser beam to continuously vary the amountof energy transmitted through the continuously variable beamsplitterwhile simultaneously oscillating the mirror in a generally sinusoidalpattern.

Example 7 (Prophetic Example)

A retroreflective sheeting could be prepared as described in Example 6,except that the optically variable mark could comprise first and secondintertwined generally sinusoidal waves and both the transmitted and thereflected beam that emerge from the continuously variable beamsplittercould be used. Turning mirrors could be used to direct both beams to thesame area. The first and second sinusoidal waves could be created bycontinuously varying the energy density of the transmitted and reflectedbeams in an opposite and complementary fashion, such that the appearanceof the two marks could vary in opposite ways.

Foreseeable modifications and alterations of this disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this invention. This invention should not be restricted tothe embodiments that are set forth in this application for illustrativepurposes.

What is claimed is:
 1. Retroreflective sheeting, comprising: astructured layer comprising cube corner elements; and at least oneoptically variable mark in the structured layer of cube corner elements;wherein the at least one optically variable mark comprises at leastthree mark elements: a first mark element, a second mark element, and athird mark element, wherein the first mark element comprises a pluralityof the cube corner elements and has a first visual feature, wherein thesecond mark element comprises a plurality of the cube corner elementsand has a second visual feature different from the first visual feature,wherein the third mark element comprises a plurality of the cube cornerelements and has a third visual feature, wherein each of the firstvisual feature, the second visual feature, and the third visual featureis retroreflectance at a given observation angle, and wherein theobservation angle required to achieve peak retroreflectance increasessequentially from the first mark element to the third mark element. 2.The retroreflective sheeting of claim 1, wherein the second visualfeature comprises a second continuously varied visual feature formed byvarying an energy density from an irradiation source applied along alength of the second continuously varied visual feature while curing thecube corner elements that form the at least one optically variable mark.3. The retroreflective sheeting of claim 1, wherein the structured layerof cube corner elements is disposed on a land layer in a range fromgreater than 0 micrometer up to 150 micrometers.
 4. The retroreflectivesheeting of claim 1, wherein the first continuously varied visualfeature and the second visual feature are independently selected from:retroreflectance at a given orientation, entrance, or observation angle;brightness at a given orientation, entrance, or observation angle;whiteness at a given orientation, entrance, or observation angle; androtational symmetry.
 5. The retroreflective sheeting of claim 1, whereinthe structured layer of the cube corner elements further comprises abackground visual appearance visually distinguishable from the at leastone optically variable mark.
 6. A security article comprising theretroreflective sheeting of claim
 1. 7. A license plate comprising theretroreflective sheeting of claim
 1. 8. A traffic sign comprising theretroreflective sheeting of claim
 1. 9. The retroreflective sheeting ofclaim 1, wherein the first continuously varied visual feature isselected from the group consisting of a shape, figure, symbol, design,letter, number, alphanumeric character, indicia, and combinationsthereof.
 10. The retroreflective sheeting of claim 1, wherein the cubecorner elements are selected from the group consisting of truncated cubecorners or full cube corners.
 11. The retroreflective sheeting of claim1, wherein the at least one optically variable mark is not visible at anobservation angle of more than ten degrees.
 12. The retroreflectivesheeting of claim 1, wherein the first mark element has a first lightdivergence profile and the second mark element has a second lightdivergence profile, visually different from the first light divergenceprofile.
 13. The retroreflective sheeting of claim 1, wherein the firstmark element includes a first set of cube corner elements and the secondmark element includes a second set of cube corner elements, and whereinthe first set of cube corner elements and second set of cube cornerelements differ in at least one of cube size, degree of cant, and cubegeometry.